This invention relates generally to a means for effecting mechanical prestressing in a pair of substantially concentric cylinders.
The provision of short-circuit proof tape windings for electrical transformers presents considerable problems when the windings are formed as annular cylinders. The cylinders must be glued together between each joint thereof in order to make the cylinders rigid. If this is insufficient the cylinders must be supported against the core legs of the yoke of the transformer. Such supports normally comprise vertical laths disposed between the core legs and the inner winding, or between inner and outer windings when two windings are used. It nevertheless becomes difficult to render such windings shortcircuit proof. During the occurrence of a short-circuit, the inner winding is pressed radially inwardly toward the core leg. However, the radial shrinkage thus arising results in a bulging of the winding cylinder at some location of its circumference, so that the conductors may break and the distance to the adjacent winding cylinder may be reduced thereby risking the occurrence of a flash-over between windings.
British Pat. No 1,184,208 discloses an arrangement for supporting a transformer winding about a core leg. Hoses of plastic, for example, are disposed at some of the inner corners of the core leg which are so defined because the core leg is built-up of packets of laminations having different widths. These hoses are arranged axially of the leg and, after the winding has been assembled over the core leg, the hoses are filled under pressure with a thermosetting agent which is thereafter cured. These hoses therefore form axially disposed supporting surfaces for the winding and are radially spaced predetermined distances apart. Thus, the winding is unsupported between these supporting surfaces so that in the event of a short-circuit, the winding between such supports may become broken inwardly and may therefore be damaged.
It is known in rotary electrical machines to stay or support winding and coil ends by means of hoses which, after assembly, are filled with a thermosetting plastic material under high pressure. If this technique is applied to effect the prestressing of transformer windings, the hose or hoses must be wound helically between the inner winding and the core. A turn of the hose will then make contact with the core leg and with the winding cylinder at different elevations around each turn. Filling of the hose with material under high pressure will therefore result in an overturning moment of forces on the winding cylinder at the ends of the winding. This moment of forces presses the winding cylinder inwardly toward the core at one side and at one end of the winding, whereas at the other end of the winding the winding cylinder is pressed inwardly toward the core leg at the opposite side. It is assumed that the winding is wound a number of whole turns around the core leg.